Semiconductor device, display device including semiconductor device, electronic device including semiconductor device, and method for manufacturing semiconductor device

ABSTRACT

A method for manufacturing a transistor with stable electric characteristics and little signal delay due to wiring resistance, used in a semiconductor device including an oxide semiconductor film. A semiconductor device including the transistor is provided. A high-performance display device including the transistor is provided.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to semiconductor devices and methods formanufacturing the same. Further, the present invention relates todisplay devices and electronic devices having the semiconductor devices.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as thin film transistor (TFT)). The transistoris applied to a wide range of electronic devices such as an integratedcircuit (IC) or an image display device (display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another material,an oxide semiconductor has been attracting attention.

For example, a technique in which a transistor is manufactured using aZn—O-based oxide or an In—Ga—Zn—O-based oxide as an oxide semiconductoris disclosed (see Patent Documents 1 and 2).

Moreover, there is a trend in a display device using a transistor (e.g.,a liquid crystal panel and an organic EL panel) toward a larger screen.As the screen size becomes larger, in the case of a display device usingan active element such as a transistor, a voltage applied to an elementvaries depending on the position of a wiring which is connected to theelement due to wiring resistance, which cause a problem of deteriorationof display quality such as display unevenness and a defect in grayscale.

In addition, a trend in resolution of a screen of the display device istoward higher definition, e.g., high-definition (HD) image quality(1366×768) or full high-definition (FHD) image quality (1920×1080), andprompt development of a 4K Digital Cinema display device, which has aresolution of 3840×2048 or 4096×2180, is also demanded.

As the resolution of the screen of the display device is improved, adriving frequency used for a driver circuit and the like in the displaydevice tends to be increased; thus, application of a low resistancematerial with little signal delay to a wiring, a signal line, or thelike is desired.

Conventionally, an aluminum film has been widely used as a material usedfor the wiring, the signal line, or the like; moreover, research anddevelopment of using a copper film as a material is extensivelyconducted to further reduce resistance. However, a copper film isdisadvantageous in that adhesion thereof to a base film is low and thatcharacteristics of a transistor easily deteriorates due to diffusion ofa copper element in the copper film into a semiconductor layer of thetransistor. Accordingly, in order to improve adhesion to the base filmand to prevent diffusion of the copper element, a technique ofmanufacturing a transistor using a silicon nitride film, a copper alloylayer formed over the silicon nitride film, and a pure copper layerformed over the copper alloy layer is disclosed (see Patent Document 3).

REFERENCE

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

[Patent Document 3] Japanese Published Patent Application No.2010-230965

SUMMARY OF THE INVENTION

Patent Document 1 assumes a silicon-based semiconductor material as amaterial for a semiconductor thin film applicable to a transistor. Thus,there has been a problem in that the disclosed technique in PatentDocument 1 is not a manufacturing method or a structure suitable for atransistor using an oxide semiconductor film in a channel formationregion.

In view of the above problems, one object of one embodiment of thepresent invention is to provide a method for manufacturing a transistorwith stable electric characteristics and little signal delay due towiring resistance, used in a semiconductor device including an oxidesemiconductor film. In addition, one object is to provide asemiconductor device including the transistor. Further, one object is toprovide a high-performance display device including the transistor.

In a method for manufacturing a semiconductor device including abottom-gate transistor using an oxide semiconductor film in a channelforming region, a source electrode and a drain electrode are formed incontact with the oxide semiconductor film. The source electrode and thedrain electrode each include a first metal film, a second metal film,and a third metal film. The second metal film uses a material containinga copper element.

A method for forming the source electrode and the drain electrode incontact with the oxide semiconductor film includes the steps of: forminga first metal film and a second metal film, performing a firstphotolithography process on the second metal film, partly removing thesecond metal film by first etching, forming the third metal film overthe first metal film and the second metal film, performing a secondphotolithography process on the third metal film, and partly removingthe first metal film and the third metal film by second etching. Thesecond etching partly removes the first metal film and the third metalfilm at an outer side of end portions of the second metal film which isremoved by the first etching. Using such a manufacturing method, thesecond metal film is covered with (preferably, surrounded by) the firstmetal film and the third metal film; thus, a material containing acopper element used in the second metal can be prevented from diffusinginto the oxide semiconductor film. Details thereof will be describedbelow.

One embodiment of the present invention is a method for manufacturing asemiconductor device including the steps of: forming a gate electrode;forming a gate insulating film over the gate electrode; forming an oxidesemiconductor film in contact with the gate insulating film and in aposition overlapping with the gate electrode; and forming a sourceelectrode and a drain electrode over the oxide semiconductor film. Inorder to form the source electrode and the drain electrode, followingsteps are included: forming a first metal film and a second metal film;performing a first photolithography process on the second metal film andpartly removing the second metal film by first etching; forming a thirdmetal film over the first metal film and the second metal film; andperforming a second photolithography process on the third metal film andpartly removing the first metal film and the third metal film by secondetching. The second etching partly removes the first metal film and thethird metal film at an outer side of end portions of the second metalfilm which is removed by the first etching.

The above method may further include the steps of: forming a firstinsulating film over the source electrode and the drain electrode;introducing oxygen into the first insulating film; forming a secondinsulating film over the first insulating film; forming an aluminum filmover the second insulating film; introducing oxygen into the aluminumfilm to form an aluminum oxide film; and forming a planarizationinsulating film over the aluminum oxide film.

Further, in each of the above-described methods, the first metal filmand the third metal film may each be a metal film or a metal nitridefilm containing one or more elements selected from tungsten, tantalum,titanium, and molybdenum. The second metal film may contain a copperelement.

Further, in each of the above-described methods, the first etching maybe a wet etching method and the second etching may be a dry etchingmethod.

Further, another embodiment of the present invention is a semiconductordevice including: a gate electrode; a gate insulating film over the gateelectrode; an oxide semiconductor film in contact with the gateinsulating film and in a position overlapping with the gate electrode;and a source electrode and a drain electrode over the oxidesemiconductor film. The source electrode and the drain electrode eachinclude a first metal film, a second metal film, and a third metal film,and the second metal film is in an inner region of end portions of thefirst metal film and the third metal film.

Further, another embodiment of the present invention is a semiconductordevice including: a gate electrode; a gate insulating film over the gateelectrode; an oxide semiconductor film in contact with the gateinsulating film and in a position overlapping with the gate electrode; asource electrode and a drain electrode over the oxide semiconductorfilm; and a signal line electrically connected to the source electrode.The signal line includes a first metal film, a second metal film, and athird metal film. The second metal film is in an inner region of endportions of the first metal film and the third metal film. The sourceelectrode and the drain electrode each include the first metal film andthe third metal film.

In the above structure, the semiconductor device further includes, overthe source electrode and the drain electrode: an oxygen-excess firstinsulating film; a second insulating film over the first insulatingfilm; an aluminum oxide film over the second insulating film; and aplanarization insulating film over the aluminum oxide film.

Further, in each of the above-described structures, the first metal filmand the third metal film may each be a metal film or a metal nitridefilm containing one or more elements selected from tungsten, tantalum,titanium, and molybdenum. In addition, the second metal film may containa copper element.

In each of the above-described structures, the gate electrode maycontain one or more elements selected from tungsten, tantalum, titanium,molybdenum, and copper.

Furthermore, the present invention also includes, in its category, adisplay device and an electronic device which include theabove-described semiconductor device.

A method for manufacturing a transistor with stable electriccharacteristics and little signal delay due to wiring resistance, usedin a semiconductor device including an oxide semiconductor film, can beprovided. Further, a semiconductor device including the transistor canbe provided. Further, a high-performance display device including thetransistor can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of one embodiment of a semiconductor device andFIGS. 1B and 1C are cross-sectional views thereof.

FIGS. 2A to 2E are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 3A to 3D are cross-sectional views illustrating an example of themanufacturing process of the semiconductor device.

FIGS. 4A to 4D are cross-sectional views illustrating an example of themanufacturing process of the semiconductor device.

FIGS. 5A to 5C are cross-sectional views illustrating an example of themanufacturing process of the semiconductor device.

FIG. 6A is a plan view of one embodiment of a semiconductor device andFIG. 6B is a cross-sectional view thereof.

FIGS. 7A to 7D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 8A and 8D are cross-sectional views illustrating an example of themanufacturing process of the semiconductor device.

FIG. 9 is a plan view illustrating one embodiment of a display device.

FIG. 10 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 11 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 12A to 12F are examples of electronic devices each including asemiconductor device.

FIGS. 13A to 13D are examples of tablet terminals each including asemiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the invention disclosed in thisspecification will be described with reference to the accompanyingdrawings. Note that the present invention is not limited to thefollowing description and it will be readily appreciated by thoseskilled in the art that modes and details can be modified in variousways without departing from the spirit and the scope of the presentinvention. Therefore, the invention should not be construed as beinglimited to the description in the following embodiments.

Note that the position, the size, the range, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for easy understanding. Therefore, thedisclosed invention is not necessarily limited to the position, size,range, or the like as disclosed in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification and the like, the term such as “over” or“below” does not necessarily mean that a component is placed “directlyon” or “directly under” another component. For example, the expression“a gate electrode over a gate insulating film” can mean the case wherethere is an additional component between the gate insulating film andthe gate electrode.

In addition, in this specification and the like, the term such as“electrode” or “wiring” does not limit a function of a component. Forexample, an “electrode” is sometimes used as part of a “wiring”, andvice versa. Furthermore, the term “electrode” or “wiring” can includethe case where a plurality of “electrodes” or “wirings” is formed in anintegrated manner.

Functions of a “source” and a “drain” are sometimes replaced with eachother when a transistor of opposite polarity is used or when thedirection of current flowing is changed in circuit operation, forexample. Therefore, the terms “source” and “drain” can be replaced witheach other in this specification and the like.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through anobject having any electric function. There is no particular limitationon an object having any electric function as long as electric signalscan be transmitted and received between components that are connectedthrough the object. Examples of an “object having any electric function”are a switching element such as a transistor, a resistor, an inductor, acapacitor, and an element with a variety of functions as well as anelectrode and a wiring.

In this specification or the like, patterning is performed by aphotolithography process. Note that the patterning is not limited to aphotolithography process and processes other than the photolithographyprocess can be employed. Further, a mask formed in the photolithographyprocess is removed after etching treatment.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and amanufacturing method thereof will be described with reference to FIGS.1A to 1C, FIGS. 2A to 2E, FIGS. 3A to 3D, FIGS. 4A to 4D, and FIGS. 5Ato 5C. In this embodiment, a transistor using an oxide semiconductorfilm is described as an example of the semiconductor device.

Structural Example 1 of Semiconductor Device

FIGS. 1A to 1C illustrate a structural example of a transistor 150. FIG.1A is a plan view of the transistor 150, FIG. 1B is a cross-sectionalview taken along the line X1-Y1 in FIG. 1A, and FIG. 1C is across-sectional view taken along the line V1-W1 in FIG. 1A. Note that inFIG. 1A, some components of the transistor 150 (e.g., a gate insulatingfilm 106) are not illustrated for clarity.

The transistor 150 illustrated in FIGS. 1A to 1C includes a gateelectrode 104 formed over a substrate 102, a gate insulating film 106formed over the gate electrode 104, an oxide semiconductor film 108formed in contact with the gate insulating film 106 and in a positionwhich overlaps with the gate electrode 104, a source electrode 110 and adrain electrode 112 formed over the oxide semiconductor film 108.

The gate electrode 104 includes a first gate electrode 104 a and asecond gate electrode 104 b. As the first gate electrode 104 a, a metalfilm or a metal nitride film containing one or more elements selectedfrom tungsten, tantalum, titanium, and molybdenum is preferably used.The second gate electrode 104 b preferably contains a copper element.For example, in this embodiment, a tungsten film is used as the firstgate electrode 104 a and a copper film is used as the second gateelectrode 104 b. With such a stacked-layer structure, the gate electrode104 can have low resistance. By providing the first gate electrode 104a, adhesion between the substrate 102 and the copper film used as thesecond gate electrode 104 b can be improved and/or diffusion of thecopper element in the copper film used as the second gate electrode 104b can be prevented.

The gate insulating film 106 includes a first gate insulating film 106 aand a second gate insulating film 106 b. It is sufficient that the firstgate insulating film 106 a has a function of preventing diffusion of thecopper element in the copper film used as the second gate electrode 104b. As the first gate insulating film 106 a, a silicon nitride film, asilicon nitride oxide film, an aluminum oxide film, an aluminum nitrideoxide film, or the like can be used. It is sufficient that the secondgate insulating film 106 b has a function of supplying oxygen to theoxide semiconductor film 108 to be formed later. As the second gateinsulating film 106 b, a silicon oxide film, a silicon oxynitride film,or the like can be used. For example, in this embodiment, a siliconnitride film is used as the first gate insulating film 106 a, and asilicon oxynitride film is used as the second gate insulating film 106b. With the gate insulating film 106 having such a stacked-layerstructure, diffusion of the copper element in the copper film used asthe gate electrode 104 can be prevented and oxygen can be supplied tothe oxide semiconductor film 108 to be formed later.

The source electrode 110 includes a first metal film 110 a, a secondmetal film 110 b, and a third metal film 110 c. The drain electrode 112includes a first metal film 112 a, a second metal film 112 b, and athird metal film 112 c. The second metal film 110 b and the second metalfilm 112 b are formed in the inner region of the end portions of thefirst metal film 110 a and the first metal film 112 a, and in the innerregion of the end portions of the third metal film 110 c and the thirdmetal film 112 c, respectively.

As the first metal film 110 a, the first metal film 112 a, the thirdmetal film 110 c, and the third metal film 112 c, a metal film or ametal nitride film containing one or more elements selected fromtungsten, tantalum, titanium, and molybdenum is preferably used. Thesecond metal film 110 b and the second metal film 112 b preferablycontain a copper element.

For example, in this embodiment, a tungsten film is used as the firstmetal film 110 a and the first metal film 112 a, a copper film is usedas the second metal film 110 b and the second metal film 112 b, and atantalum nitride film is used as the third metal film 110 c and thethird metal film 112 c. The second metal film 110 b and the second metalfilm 112 b are formed over the first metal film 110 a and the firstmetal film 112 a and covered with the third metal film 110 c and thethird metal film 112 c, respectively.

That is to say, a bottom surface of the copper film used as the secondmetal film 110 b and the second metal film 112 b is covered with thetungsten film used as the first metal film 110 a and the first metalfilm 112 a and a top surface and side surfaces thereof are covered withthe tantalum nitride film used as the third metal film 110 c and thethird metal film 112 c. The first metal film 110 a, the first metal film112 a, the third metal film 110 c, and the third metal film 112 c have afunction as a barrier metal for preventing diffusion of the copperelement in the copper film.

With the source electrode 110 and the drain electrode 112 having suchstructures, the source electrode 110 and the drain electrode 112 canhave low resistance and diffusion of the copper element in the copperfilm used in the source electrode 110 and the drain electrode 112 to theoutside can be prevented.

As a method for forming the source electrode 110 and the drain electrode112, for example, the first metal film and the second metal film areformed over the oxide semiconductor film 108, first photolithographyprocess is performed on the second metal film, and first etching isperformed so that the second metal film is partly removed, whereby thesecond metal film 110 b and the second metal film 112 b are formed.Then, the third metal film is formed over the first metal film and thesecond metal film (the second metal film 110 b and the second metal film112 b) so as to cover the second metal film. Then, secondphotolithography process is performed on the third metal film, and thefirst metal film and the third metal film are partly removed by secondetching, whereby the first metal film 110 a, the first metal film 112 a,the third metal film 110 c, and the third metal film 112 c are formed.With such a formation method, the copper film used as the second metalfilm is not directly in contact with the oxide semiconductor film 108,so that diffusion of impurities (in particular, a copper element) whichmight enter a back channel portion of the oxide semiconductor film 108can be prevented.

Furthermore, a structure that includes an oxygen-excess first insulatingfilm 114 a over the source electrode 110 and the drain electrode 112, asecond insulating film 114 b formed over the first insulating film 114a, an aluminum oxide film 116 formed over the second insulating film 114b, and a planarization insulating film 118 formed over the aluminumoxide film 116 may also be used.

Note that the details of the other components will be described below inan example of a method for manufacturing the transistor 150 illustratedin FIGS. 1A to 1C, with reference to FIGS. 2A to 2E, FIGS. 3A to 3D,FIGS. 4A to 4D, and FIGS. 5A to 5C.

Method 1 for Manufacturing Semiconductor Device

First, the gate electrode 104 including the first gate electrode 104 aand the second gate electrode 104 b is formed over the substrate 102(see FIG. 2A).

Although there is no particular limitation on a substrate that can beused as the substrate 102, it is necessary that the substrate have heatresistance to withstand at least a heat treatment performed later. Forexample, a variety of glass substrates for electronics industry, such asa barium borosilicate glass substrate or an aluminoborosilicate glasssubstrate can be used. Note that as the substrate, a glass substratehaving a coefficient of thermal expansion which is greater than or equalto 25×10⁻⁷/° C. and less than or equal to 50×10⁻⁷/° C. (preferably,greater than or equal to 30×10⁻⁷/° C. and less than or equal to40×10⁻⁷/° C.) and a strain point which is higher than or equal to 650°C. and lower than or equal to 750° C. (preferably, higher than or equalto 700° C. and lower than or equal to 740° C.) is preferably used.

In the case where a large-sized glass substrate having the size of thefifth generation (1000 mm×1200 mm or 1300 mm'1500 mm), the sixthgeneration (1500 mm×1800 mm), the seventh generation (1870 mm×2200 mm),the eighth generation (2200 mm×2500 mm), the ninth generation (2400mm×2800 mm), the tenth generation (2880 mm×3130 mm), or the like isused, minute processing might become difficult owing to shrinkage of thesubstrate caused by heat treatment or the like in the manufacturingprocess of a semiconductor device. Therefore, in the case where theabove-described large-sized glass substrate is used as the substrate, asubstrate with little shrinkage is preferably used. For example, thesubstrate can be a large-sized glass substrate in which, after heattreatment which is performed for one hour at preferably 450° C., morepreferably 500° C., the amount of shrinkage is less than or equal to 20ppm, preferably less than or equal to 10 ppm, more preferably less thanor equal to 5 ppm.

Alternatively, the semiconductor device may be manufactured using aflexible substrate as the substrate 102. In order to manufacture aflexible semiconductor device, the transistor 150 including the oxidesemiconductor film 108 may be directly formed over a flexible substrate,or the transistor 150 including the oxide semiconductor film 108 may beformed over a manufacturing substrate, and then, the transistor 150 maybe separated from the manufacturing substrate and transferred to aflexible substrate. Note that in order to separate the transistor 150from the manufacturing substrate and transfer it to the flexiblesubstrate, a separation layer may be provided between the manufacturingsubstrate and the transistor 150 including the oxide semiconductor film.

Further, a base insulating film may be provided over the substrate 102.The base insulating film can be formed by a plasma CVD method, asputtering method, or the like using an oxide insulating film such as asilicon oxide film, a silicon oxynitride film, an aluminum oxide film,an aluminum oxynitride film, a hafnium oxide film, or a gallium oxidefilm; a nitride insulating film such as a silicon nitride film, asilicon nitride oxide film, an aluminum nitride film, or an aluminumnitride oxide film; or a mixed material thereof.

Further, the substrate 102 is preferably subjected to heat treatment.For example, the heat treatment may be performed with a gas rapidthermal annealing (GRTA) apparatus, in which heat treatment is performedusing a high-temperature gas, at 650° C. for 1 minute to 5 minutes. Asthe high-temperature gas for GRTA, an inert gas which does not reactwith an object to be processed by heat treatment, such as nitrogen or arare gas like argon, is used. Alternatively, the heat treatment may beperformed with an electric furnace at 500° C. for 30 minutes to onehour.

The gate electrode 104 can be formed using a material including one ormore elements selected from tungsten, tantalum, titanium, molybdenum,and copper. In this embodiment, as the second gate electrode 104 b, acopper film with a thickness greater than or equal to 100 nm and lessthan or equal to 400 nm is formed by a sputtering method. Moreover, as alayer under the second gate electrode 104 b, the first gate electrode104 a which functions as a barrier metal for preventing diffusion of thecopper element in the copper film is formed. In this embodiment, as thefirst gate electrode 104 a, a tantalum nitride film with a thicknessgreater than or equal to 20 nm and less than or equal to 100 nm isformed by a sputtering method.

Note that the stacked-layer structure of the first gate electrode 104 aand the second gate electrode 104 b is described in this embodiment;however, the present invention is not limited to this structure. Forexample, a third gate electrode may be provided over the second gateelectrode 104 b. As the third gate electrode, a material similar to thatof the first gate electrode 104 a can be used.

Next, the gate insulating film 106 including the first gate insulatingfilm 106 a and the second gate insulating film 106 b is formed over thesubstrate 102 and the gate electrode 104 (see FIG. 2B).

For the first gate insulating film 106 a, a nitride insulating filmformed by a plasma CVD method, a sputtering method, or the like ispreferably used. The thickness of the nitride insulating film ispreferably greater than or equal to 10 nm and less than or equal to 100nm, further preferably greater than or equal to 20 nm and less than orequal to 50 nm. For example, a silicon nitride film and a siliconnitride oxide film can be given. A nitride insulating film is used asthe first gate insulating film 106 a which is in contact with thesubstrate 102 and the gate electrode 104, thereby preventing diffusionof impurities from the substrate 102 and the gate electrode 104. Inparticular, in the case where a metal material containing a copperelement is used as the gate electrode 104 (more specifically, the secondgate electrode 104 b), the first gate insulating film 106 a can preventdiffusion of the copper element to the oxide semiconductor film 108.

In this embodiment, as the first gate insulating film 106 a, a50-nm-thick silicon nitride film formed by a plasma CVD method is used.As a gas used for forming the silicon nitride film, a mixed gas ofsilane (SiH₄) and nitrogen, a mixed gas of silane, nitrogen, and ammonia(NH₃), or the like can be used.

For the second gate insulating film 106 b, an oxide insulating filmformed by a plasma CVD method, a sputtering method, or the like ispreferably used. The thickness of the oxide insulating film ispreferably greater than or equal to 100 nm and less than or equal to 350nm, further preferably greater than or equal to 100 nm and less than orequal to 200 nm. For example, a silicon oxide film, a gallium oxidefilm, an aluminum oxide film, a silicon oxynitride film, an aluminumoxynitride film can be given.

The second gate insulating film 106 b can be formed using a high-kmaterial such as a hafnium oxide film, an yttrium oxide film, a hafniumsilicate film (HfSi_(x)O_(y) (x>0, y>0)), a hafnium silicate film towhich nitrogen is added (HfSiO_(x)N_(y) (x>0, y>0)), a hafnium aluminatefilm (HfAl_(x)O_(y) (x>0, y>0)), or a lanthanum oxide film, whereby gateleakage current can be reduced.

In this embodiment, as the second gate insulating film 106 b, a200-nm-thick silicon oxynitride film formed by a plasma CVD method isused. The film formation time in the case of using a plasma CVD methodcan be made shorter than that of the case of using a sputtering method.Further, with a plasma CVD method, variation in the film thickness ofthe second gate insulating film 106 b which is formed is smaller andentry of particles into the plane thereof occurs less frequently thanthose in the case of using a sputtering method.

Note that since the second gate insulating film 106 b is an insulatingfilm in contact with the oxide semiconductor film 108, it is preferableto contain oxygen and also preferable to contain impurities such aswater or hydrogen as little as possible. However, in the case of using aplasma CVD method, it is more difficult to reduce the concentration ofhydrogen in the layer than the case of using a sputtering method. Thus,heat treatment for reducing (preferably, removing) hydrogen atoms (thetreatment is dehydration or dehydrogenation treatment) may be performedon the second gate insulating film 106 b.

The heat treatment is performed at higher than or equal to 250° C. andlower than or equal to 650° C., preferably higher than or equal to 450°C. and lower than or equal to 600° C. or lower than the strain point ofthe substrate. For example, the substrate is introduced into an electricfurnace which is one of heat treatment apparatuses, and heat treatmentis performed on the gate insulating film 106 at 650° C. for one hour invacuum (under reduced pressure).

Note that the heat treatment apparatus is not limited to the electricfurnace, and an apparatus for heating an object by heat conduction orheat radiation from a heater such as a resistance heater may be used.For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gasrapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal)apparatus can be used. An LRTA apparatus is an apparatus for heating anobject to be processed by radiation of light (an electromagnetic wave)emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenonarc lamp, a carbon arc lamp, a high pressure sodium lamp, or a highpressure mercury lamp. A GRTA apparatus is an apparatus for performingheat treatment using a high temperature gas. As the high temperaturegas, an inert gas which does not react with an object by heat treatment,such as nitrogen or a rare gas like argon, is used. Note that in thecase where a GRTA apparatus is used as the heat treatment apparatus, thesubstrate may be heated in an inert gas heated to a high temperature of650° C. to 700° C. because the heat treatment time is short.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which a water content is 20 ppm or less,preferably 1 ppm or less, further preferably 10 ppb or less), or a raregas (argon, helium, or the like). Note that it is preferable that water,hydrogen, and the like be not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas. It is also preferable that thepurity of nitrogen, oxygen, or the rare gas which is introduced into aheat treatment apparatus be set to be 6N (99.9999%) or higher,preferably 7N (99.99999%) or higher (that is, the impurity concentrationis 1 ppm or lower, preferably 0.1 ppm or lower).

With the heat treatment, dehydration or dehydrogenation can be performedon the gate insulating film 106, so that the gate insulating film 106from which impurities such as hydrogen and water causing variation inthe characteristics of the transistor are removed can be formed.

The heat treatment for dehydration or dehydrogenation may be performedplural times, and may also serve as another heat treatment.

Next, the oxide semiconductor film 108 is formed in contact with thegate insulating film 106 so as to overlap with the gate electrode 104(see FIG. 2C).

The oxide semiconductor film 108 may have either a single-layerstructure or a stacked-layer structure. Further, the oxide semiconductorlayer may either have an amorphous structure or a crystalline structure.In the case where the oxide semiconductor film 108 has an amorphousstructure, heat treatment may be performed on the oxide semiconductorfilm 108 in a later manufacturing step so that the oxide semiconductorfilm has crystallinity. The heat treatment for crystallizing theamorphous oxide semiconductor film is performed at a temperature higherthan or equal to 250° C. and lower than or equal to 700° C., preferablyhigher than or equal to 400° C., further preferably higher than or equalto 500° C., still further preferably higher than or equal to 550° C.Note that the heat treatment can also serve as another heat treatment inthe manufacturing process.

The oxide semiconductor film 108 can be formed by a sputtering method, amolecular beam epitaxy (MBE) method, a plasma CVD method, a pulse laserdeposition method, an atomic layer deposition (ALD) method, or the likeas appropriate.

When the oxide semiconductor film 108 is formed, it is preferable thatthe concentration of hydrogen contained in the oxide semiconductor film108 be reduced as much as possible. In order to reduce the hydrogenconcentration, for example, in the case where a sputtering method isused for film formation, a high-purity rare gas (typically, argon) fromwhich impurities such as hydrogen, water, a hydroxyl group, and ahydride have been removed; oxygen; or a mixed gas of oxygen and the raregas is used as appropriate as an atmosphere gas supplied to a treatmentchamber of a sputtering apparatus.

The oxide semiconductor film 108 is formed in such a manner that asputtering gas from which hydrogen and water have been removed isintroduced into the treatment chamber while moisture remaining in thetreatment chamber is removed, whereby the hydrogen concentration in theformed oxide semiconductor film 108 can be reduced. In order to removemoisture remaining in the treatment chamber, an entrapment vacuum pumpsuch as a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. A turbo molecular pump provided with a cold trap may beused. The cryopump has a high capability in removing a hydrogenmolecule, a compound containing a hydrogen atom such as water (H₂O)(more preferably, also a compound containing a carbon atom), and thelike; thus, the impurity concentration in the oxide semiconductor film108 formed in the treatment chamber which is evacuated with the cryopumpcan be reduced.

Note that in this embodiment, the oxide semiconductor film 108 is formedby a sputtering method using a metal oxide target with an atomic ratioof In:Ga:Zn=1:1:1 or a metal oxide target with an atomic ratio ofIn:Ga=2:1. Note that the target that can be used for forming the oxidesemiconductor film 108 is not limited to the target including the abovematerials with the above ratios. Further, the oxide semiconductor film108 can be formed by a sputtering method in a rare gas (typically argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gasand oxygen. Further, the target that can be used for forming the oxidesemiconductor film 108 preferably has crystallinity; that is, a singlecrystalline target, a polycrystalline target, or the like are preferablyused. With the use of the target having crystallinity, a formed thinfilm also has crystallinity; specifically, the formed thin film tends tohave a c-axis-aligned crystal.

In addition, the oxide semiconductor film 108 immediately after beingformed is preferably in a supersaturated state where the amount ofoxygen exceeds the amount of oxygen in the stoichiometric composition.For example, when the oxide semiconductor film 108 is formed by asputtering method, it is preferable that the film be formed in a filmformation gas containing a high percentage of oxygen, and it isespecially preferable that the film be formed under an oxygen atmosphere(oxygen gas 100%). For example, when the oxide semiconductor film 108 isformed using an In—Ga—Zn-based oxide (IGZO) under a condition that theproportion of oxygen in the film formation gas is large (in particular,oxygen gas: 100%), Zn release from the film can be suppressed even whenthe film formation temperature is 300° C. or higher.

Further, when the oxide semiconductor film 108 is formed using the abovemetal oxide target with the atomic ratio of In:Ga:Zn=1:1:1, thecomposition of the target is different from the composition of a thinfilm formed over the substrate in some cases. For example, when themetal oxide target with the atomic ratio of In:Ga:Zn=1:1:1 is used, thecomposition ratio of the oxide semiconductor film 108, which is the thinfilm, becomes In:Ga:Zn=1:1:0.6 to 1:1:0.8 in an atomic ratio in somecases, though it depends on the film formation conditions. This isbecause in formation of the oxide semiconductor film 108, Zn issublimed, or because a sputtering rate differs between the components ofIn, Ga, and Zn.

Accordingly, when a thin film having a preferable composition ratio isformed, a composition ratio of the metal oxide target needs to beadjusted in advance. For example, in order to make the composition ratioof the thin oxide semiconductor film 108 be In:Ga:Zn=1:1:1 in an atomicratio, the composition ratio of the metal oxide target is made to beIn:Ga:Zn=1:1:1.5 in an atomic ratio. In other words, the contentpercentage of Zn in the metal oxide target is preferably made higher inadvance. The composition ratio of the target is not limited to the abovevalue, and can be adjusted as appropriate depending on the filmformation conditions or the composition of the thin film to be formed.Further, it is preferable to increase the content percentage of Zn inthe metal oxide target because in that case, the obtained thin film canhave higher crystallinity.

Further, in the case where the oxide semiconductor film 108 is formed bya sputtering method, the relative density of the metal oxide targetwhich is used for forming the oxide semiconductor film 108 is greaterthan or equal to 90% and less than or equal to 100%, preferably greaterthan or equal to 95%, more preferably greater than or equal to 99.9%.With the use of the metal oxide target with a high relative density, theformed oxide semiconductor film 108 can be a dense film.

In order to reduce the impurity concentration in the oxide semiconductorfilm 108, it is also effective to form the oxide semiconductor film 108while the substrate 102 is kept at high temperature. The heatingtemperature of the substrate 102 may be higher than or equal to 150° C.and lower than or equal to 450° C., preferably higher than or equal to170° C. and lower than or equal to 350° C. By heating the substrate at ahigh temperature during the film formation, the oxide semiconductor film108 having crystallinity can be formed.

An oxide semiconductor to be used for the oxide semiconductor film 108preferably contains at least indium (In) or zinc (Zn). In particular,both In and Zn are preferably contained. As a stabilizer for reducingvariation in electric characteristics of a transistor including theoxide semiconductor, gallium (Ga) is preferably additionally contained.Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf) ispreferably contained as a stabilizer. Aluminum (Al) is preferablycontained as a stabilizer. Zirconium (Zr) is preferably contained as astabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

As the oxide semiconductor, for example, indium oxide, tin oxide, zincoxide, an In—Zn-based oxide, a Sn-Zn-based oxide, an Al—Zn-based oxide,a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, anIn—Ga-based oxide, an In—Ga—Zn-based oxide (also referred to as IGZO),an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-basedoxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, anIn—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide,an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-basedoxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, anIn—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide,an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-basedoxide, or an In—Lu—Zn-based oxide, an In—Sn—Ga—Zn-based oxide, anIn—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or anIn—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an “In—Ga—Zn—O-based oxide” means an oxidecontaining In, Ga, and Zn as its main component and there is noparticular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxidemay contain a metal element other than the In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)m (m>0 is satisfied,and m is not an integer) may be used as an oxide semiconductor. Notethat M represents one or more metal elements selected from Ga, Fe, Mn,and Co. Alternatively, as the oxide semiconductor, a materialrepresented by In₂SnO₅(ZnO)_(n) (n>0 is satisfied, n is an integer) maybe used.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=2:2:1 (=2/5:2/5:1/5),In:Ga:Zn=3:1:2 (=1/2:1/6:1/3), or any of oxides whose composition is inthe neighborhood of the above compositions can be used. Alternatively,an In—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1(=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5(=1/4:1/8:5/8), or any of oxides whose composition is in theneighborhood of the above compositions may be used.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used depending on neededsemiconductor characteristics (e.g., mobility, threshold voltage, andvariation). In order to obtain the required semiconductorcharacteristics, it is preferable that the carrier concentration, theimpurity concentration, the defect density, the atomic ratio between ametal element and oxygen, the interatomic distance, the density, and thelike be set to appropriate values.

For example, high mobility can be obtained relatively easily in the caseof using an In—Sn—Zn oxide. However, mobility can be increased byreducing the defect density in a bulk also in the case of using anIn—Ga—Zn-based oxide.

For example, in the case where the composition of an oxide containingIn, Ga, and, Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1), is in theneighborhood of the composition of an oxide containing In, Ga, and, Znat the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1), a, b, and c satisfy thefollowing relation: (a-A)²+(b-B)²+(c-C)²≦and r may be 0.05, for example.The same applies to other oxides.

The oxide semiconductor film 108 is preferably a c-axis alignedcrystalline oxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor layer with acrystal-amorphous mixed phase structure where crystalline portions areincluded in an amorphous phase. Note that in most cases, the crystalpart fits inside a cube whose one side is less than 100 nm. From anobservation image obtained with a transmission electron microscope(TEM), a boundary between an amorphous part and a crystal part in theCAAC-OS film is not clear. Further, with the TEM, a grain boundary inthe CAAC-OS film is not found. Thus, in the CAAC-OS film, a reduction inelectron mobility, due to the grain boundary, is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystal parts, thedirections of the a-axis and the b-axis of one crystal part may bedifferent from those of another crystal part. In this specification, asimple term “perpendicular” includes a range from 85° to 95°. Inaddition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystal part in a region to which the impurity is added becomesamorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note thatwhen the CAAC-OS film is formed, the direction of c-axis of the crystalpart is the direction parallel to a normal vector of the surface wherethe CAAC-OS film is formed or a normal vector of the surface of theCAAC-OS film. The crystal part is formed by film formation or byperforming treatment for crystallization such as heat treatment afterfilm formation.

With the use of the CAAC-OS film in a transistor, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small. Thus, the transistor has highreliability.

There are three methods for obtaining a CAAC-OS film when the CAAC-OSfilm is used as the oxide semiconductor film 108. The first method is toform an oxide semiconductor layer at a film formation temperature higherthan or equal to 100° C. and lower than or equal to 450° C., morepreferably higher than or equal to 150° C. and lower than or equal to400° C., thereby obtaining c-axis alignment substantially perpendicularto a surface. The second method is to form a thin oxide semiconductorfilm and then subject the film to heat treatment performed at atemperature higher than or equal to 200° C. and lower than or equal to700° C., thereby obtaining c-axis alignment substantially perpendicularto a surface. The third method is to form a first thin oxidesemiconductor film, subject the film to heat treatment performed at atemperature higher than or equal to 200° C. and lower than or equal to700° C., and then form a second oxide semiconductor film, therebyobtaining c-axis alignment substantially perpendicular to a surface.

Note that when an oxide semiconductor film having crystallinitydifferent from the CAAC-OS film (single crystal or microcrystalline) isformed as the oxide semiconductor film 108, the film formationtemperature is not particularly limited.

The energy gap of the oxide semiconductor film 108 is 2.8 eV to 3.2 eV,and is greater than that of silicon (1.1 eV). The intrinsic carrierdensity of the oxide semiconductor film 108 is 10⁻⁹ cm⁻³, which is muchsmaller than the intrinsic carrier density of silicon (10¹¹ cm⁻³).

Majority carriers (electrons) of the oxide semiconductor film 108 flowonly from a source of a transistor. Further, a channel formation regioncan be depleted completely. Thus, an off-state current of the transistorcan be extremely small. The off-state current of the transistorincluding the oxide semiconductor film 108 is as small as 10 yA/μm orless at room temperature, and 1 zA/μm or less at 85° C. to 95° C.

Note that the oxide semiconductor film 108 may have a structure in whicha plurality of oxide semiconductor layers are stacked. For example, theoxide semiconductor film 108 may have a stacked-layer structure of afirst oxide semiconductor layer and a second oxide semiconductor layerwhich are formed using metal oxides with different compositions. Forexample, the first oxide semiconductor layer may be formed using athree-component metal oxide, and the second oxide semiconductor layermay be formed using a two-component metal oxide. Alternatively, both thefirst oxide semiconductor layer and the second oxide semiconductor layermay be formed using a three-component metal oxide.

Further, the constituent elements of the first oxide semiconductor layerand the second oxide semiconductor layer may be made to be the same andthe composition of the constituent elements may be made to be differentfrom each other. For example, the first oxide semiconductor layer mayhave an atomic ratio of In:Ga:Zn=1:1:1 and the second oxidesemiconductor layer may have an atomic ratio of In:Ga:Zn=3:1:2.Alternatively, the first oxide semiconductor layer may have an atomicratio of In:Ga:Zn=1:3:2 and the second oxide semiconductor layer mayhave an atomic ratio of In:Ga:Zn=2:1:3.

At this time, one of the first oxide semiconductor layer and the secondoxide semiconductor layer which is closer to the gate electrode (on achannel side) preferably contains In and Ga at a proportion of In>Ga.The other which is farther from the gate electrode (on a back channelside) preferably contains In and Ga at a proportion of In≦Ga. In anoxide semiconductor, the s orbital of heavy metal mainly contributes tocarrier transfer, and when the In content in the oxide semiconductor isincreased, overlap of the s orbital is likely to be increased.Therefore, an oxide having a composition of In>Ga has higher mobilitythan an oxide having a composition of In≦Ga. Further, in Ga, theformation energy of oxygen vacancy is larger and thus oxygen vacancy isless likely to occur, than in In; therefore, the oxide having acomposition of In≦Ga has more stable characteristics than the oxidehaving a composition of In>Ga. Thus, an oxide semiconductor layercontaining In and Ga at a proportion of In>Ga is used on a channel side,and an oxide semiconductor layer containing In and Ga at a proportion ofIn≦Ga is used on a back channel side; so that the mobility andreliability of the transistor can be further improved.

Further, when the oxide semiconductor film 108 has a stacked-layerstructure, the first oxide semiconductor layer and the second oxidesemiconductor layer may be formed using oxide semiconductors havingdifferent crystallinity. That is, the oxide semiconductor film 108 mayhave a structure in which two of a single crystal oxide semiconductor, apolycrystalline oxide semiconductor, an amorphous oxide semiconductor,and an oxide semiconductor having crystallinity (for example, CAAC-OS)are combined as appropriate. When an amorphous oxide semiconductor isused for at least one of the first oxide semiconductor layer and thesecond oxide semiconductor layer, internal stress or external stress ofthe oxide semiconductor is relieved, variation in characteristics of atransistor is reduced, and reliability of the transistor can be furtherimproved. On the other hand, in the amorphous oxide semiconductor, animpurity acting as a donor, such as hydrogen, is easily absorbed andoxygen deficiency easily occur; thus, the amorphous oxide semiconductoris likely to be n-type. Therefore, it is preferable that the oxidesemiconductor having crystallinity (for example, CAAC-OS) be used forthe oxide semiconductor layer on the channel side.

In the case of the oxide semiconductor film 108 having a stacked-layerstructure, the stacked-layer structure with the following compositionand combination of crystallinity can be given: a stacked-layer structurein which an amorphous oxide semiconductor layer with an atomic ratio inthe neighborhood of In:Ga:Zn=1:1:1 and a crystalline oxide semiconductorlayer with an atomic ratio in the neighborhood of In:Ga:Zn=3:1:2 whichare stacked over the gate insulating film 106 in this order, or astacked-layer structure in which a crystalline oxide semiconductor layerwith an atomic ratio in the neighborhood of In:Ga:Zn=1:1:1 and acrystalline oxide semiconductor layer with an atomic ratio in theneighborhood of In:Ga:Zn=3:1:2 which are stacked over the gateinsulating film 106 in this order. Other than the above stacked-layerstructures, the following may be employed: a stacked-layer structure inwhich a crystalline oxide semiconductor layer with an atomic ratio inthe neighborhood of In:Ga:Zn=3:1:2 and a crystalline oxide semiconductorlayer with an atomic ratio in the neighborhood of In:Ga:Zn=1:1:1, astacked-layer structure in which an amorphous oxide semiconductor layerwith an atomic ratio in the neighborhood of In:Ga:Zn=1:1:1 and anamorphous oxide semiconductor layer with an atomic ratio in theneighborhood of In:Ga:Zn=3:1:2, or a stacked-layer structure in which anamorphous oxide semiconductor layer with an atomic ratio in theneighborhood of In:Ga:Zn=3:1:2 and an amorphous oxide semiconductorlayer with an atomic ratio in the neighborhood of In:Ga:Zn=1:1:1.

Before the oxide semiconductor film 108 is formed, planarizationtreatment may be performed on the surface on which the oxidesemiconductor film 108 is to be formed. As the planarization treatment,polishing treatment (e.g., chemical mechanical polishing (CMP)),dry-etching treatment, or plasma treatment can be used, though there isno particular limitation on the planarization treatment.

As plasma treatment, reverse sputtering in which an argon gas isintroduced and plasma is generated can be performed. The reversesputtering is a method in which voltage is applied to a substrate sidewith the use of an RF power source in an argon atmosphere and plasma isgenerated in the vicinity of the substrate so that a substrate surfaceis modified. Note that instead of argon, nitrogen, helium, oxygen or thelike may be used. The reverse sputtering can remove particle substances(also referred to as particles or dust) attached to the surface on whichthe oxide semiconductor film 108 is to be formed.

As the planarization treatment, polishing treatment, dry etchingtreatment, or plasma treatment may be performed plural times, or thesetreatments may be performed in combination. In the case where thetreatments are combined, the order of steps is not particularly limitedand may be set as appropriate depending on the roughness of the surfaceon which the oxide semiconductor film 108 is to be formed.

Further, after the oxide semiconductor film 108 is formed, the oxidesemiconductor film 108 is preferably subjected to heat treatment forreducing or removing excess hydrogen (including water and a hydroxylgroup) contained in the oxide semiconductor film 108 (dehydration ordehydrogenation). The heat treatment can be performed in a conditionsimilar to that of the heat treatment of the second gate insulating film106 b described above.

The heat treatment enables reduction, more preferably removal ofhydrogen, which is an impurity imparting n-type conductivity, in theoxide semiconductor film 108. Further, in the case where an insulatingfilm containing oxygen is used as the second gate insulating film 106 b,by this heat treatment, oxygen contained in the second gate insulatingfilm 106 b can be supplied to the oxide semiconductor film 108. Whileoxygen is released from the oxide semiconductor film 108 by thedehydration or dehydrogenation treatment, oxygen is supplied from thesecond gate insulating film 106 b to the oxide semiconductor film 108,whereby oxygen vacancies in the oxide semiconductor film 108 can befilled.

Further, after the oxide semiconductor film 108 is heated through theheat treatment, a high-purity oxygen gas, a high-purity dinitrogenmonoxide gas, or ultra dry air (the moisture amount is less than orequal to 20 ppm (−55° C. by conversion into a dew point), preferablyless than or equal to 1 ppm, more preferably less than or equal to 10ppb, in the measurement with the use of a dew-point instrument of acavity ring down laser spectroscopy (CRDS) system) may be introducedinto the same furnace while the heating temperature is maintained orgradually decreased. It is preferable that water, hydrogen, or the likebe not contained in the oxygen gas or the dinitrogen monoxide gas.Alternatively, the purity of the oxygen gas or the dinitrogen monoxidegas which is introduced into the heat treatment apparatus is preferably6N or higher, further preferably 7N or higher (i.e., the impurityconcentration in the oxygen gas or the dinitrogen monoxide gas ispreferably 1 ppm or lower, further preferably 0.1 ppm or lower). Whileoxygen is reduced by removing an impurity for the dehydration ordehydrogenation, the oxygen gas or the dinitrogen monoxide gas acts tosupply oxygen that is a main component of the oxide semiconductor film108, so that the oxide semiconductor film 108 can have high purity andbe an i-type (intrinsic) oxide semiconductor film.

The heat treatment for dehydration or dehydrogenation may serve asanother heat treatment of a manufacturing process of the transistor 150.

Next, a first metal film 109 a and a second metal film 109 b to be asource electrode and a drain electrode (as well as a wiring formed inthe same layer as the source electrode and the drain electrode) areformed over the gate insulating film 106 and the oxide semiconductorfilm 108 (see FIG. 2D).

The first metal film 109 a is preferably a metal film or a metal nitridefilm containing one or more elements selected from tungsten, tantalum,titanium, and molybdenum. In this embodiment, as the first metal film109 a, a 50-nm-thick tungsten film formed by a sputtering method isused.

Further, the first metal film 109 a may have a stacked-layer structure.For example, the stacked-layer structure of the first metal film 109 aincludes a metal film containing one or more elements selected fromtungsten, tantalum, titanium, and molybdenum as a first layer, and ametal nitride film containing one or more elements selected fromtungsten nitride, tantalum nitride, titanium nitride and molybdenumnitride as a second layer.

For the first metal film 109 a, since the first metal film 109 a is incontact with the oxide semiconductor film 108, a material which does notextract oxygen from the oxide semiconductor film 108 and does not makethe oxide semiconductor film 108 have n-type conductivity, or a materialwhich is not diffused into the oxide semiconductor film 108 and does notmake the oxide semiconductor film 108 have n-type conductivity is used.In addition, a material which prevents diffusion of a copper elementfrom a copper film used as the second metal film 109 b to the oxidesemiconductor film 108 (such a material is called as a barrier metalmaterial) is preferably used for the first metal film 109 a.

The second metal film 109 b preferably contains a copper element. Notethat a copper alloy or the like in which aluminum, gold, silver, zinc,tin, nickel, or the like is added at several weight percent to coppermay be used. In this embodiment, as the second metal film 109 b, a200-nm-thick copper film formed by a sputtering method is used.

Next, a resist is applied over the second metal film 109 b and the firstpatterning is performed, so that a resist mask 141 is formed (see FIG.2E).

The resist mask 141 can be formed in such a manner that a photosensitiveresin is applied, and then the photosensitive resin is exposed anddeveloped. Note that the photosensitive resin may be a negative-type orpositive-type photosensitive resin.

Alternatively, the resist mask 141 may be formed by an inkjet method, inwhich case manufacturing costs can be reduced because a photomask is notused.

Next, the second metal film 109 b is partly removed by the firstetching, so that the second metal film 110 b and the second metal film112 b are formed (see FIG. 3A).

As a method for removing the second metal film 109 b, a wet etchingmethod is preferable. As a chemical solution used in the wet etchingmethod, it is preferable that a chemical solution which can etch thesecond metal film 109 b and does not remove the first metal film 109 abe used. For example, in the case where a tungsten film is used as thefirst metal film 109 a and a copper film is used as the second metalfilm 109 b, a mixed solution of water, hydrogen peroxide water, andcarboxylic acid; a mixed solution of water, phosphoric acid, nitricacid, sulfuric acid, and potassium sulfate; or the like can be used asthe chemical solution.

Further, by adjusting a wet etching time and performing isotropicetching, the side surfaces of the second metal film 110 b and the secondmetal film 112 b may be receded to be at the inner side of the sidesurfaces of the resist mask 141.

Next, the resist mask 141 is removed (see FIG. 3B).

As a method for removing the resist mask 141, a wet removing methodusing a stripping solution, a dry removing method such as plasmatreatment, a combination thereof, or the like can be used.

Next, a third metal film 109 c is formed over the first metal film 109a, the second metal film 110 b, and the second metal film 112 b (seeFIG. 3C).

The third metal film 109 c can be formed using a method and a materialsimilar to those of the first metal film 109 a. Note that in thisembodiment, as the third metal film 109 c, a 100-nm-thick tantalumnitride film formed by a sputtering method is used.

Next, a resist is applied over the third metal film 109 c, and thesecond patterning is performed, whereby a resist mask 142 is formed (seeFIG. 3D).

The resist mask 142 can be formed using a material and a method similarto those of the resist mask 141.

Next, the first metal film 109 a and the third metal film 109 c arepartly removed by the second etching, whereby the first metal film 110a, the first metal film 112 a, the third metal film 110 c, and the thirdmetal film 112 c are formed (see FIG. 4A).

Note that the first metal film 109 a and the third metal film 109 c arepartly removed by the second etching so that the removed portions are atthe outer side of the end portions of the second metal film 110 b andthe second metal film 112 b which are removed by the first etching.

As a method for removing the first metal film 109 a and the third metalfilm 109 c, a dry etching method is preferably used. As a gas used forthe dry etching method, for example, in the case where a tungsten filmis used as the first metal film 109 a and a tantalum nitride film isused as the third metal film 109 c, a mixed gas containing SF₆ and O₂, amixed gas containing SF₆ and BCl₃, or the like can be used.

Note that it is desirable that etching conditions be optimized so as notto etch and divide the oxide semiconductor film 108 when the first metalfilm 109 a and the third metal film 109 c are etched. However, it isdifficult to obtain etching conditions in which only the first metalfilm 109 a and the third metal film 109 c are etched and the oxidesemiconductor film 108 is not etched at all. In some cases, when thefirst metal film 109 a and the third metal film 109 c are etched, theoxide semiconductor film 108 is partly etched to have a groove portion(a recessed portion).

Next, the resist mask 142 is removed, and the source electrode 110including the first metal film 110 a, the second metal film 110 b, andthe third metal film 110 c and the drain electrode 112 including thefirst metal film 112 a, the second metal film 112 b, and the third metalfilm 112 c are formed (see FIG. 4B).

With such a method of forming the source electrode 110 and the drainelectrode 112, the oxide semiconductor film 108 (more specifically, theback channel side thereof) is not in contact with the copper films usedas the second metal film 110 b and the second metal film 112 b; thus, itis possible to prevent a copper element from attaching or diffusing tothe oxide semiconductor film 108.

Note that the resist mask 142 can be removed by a method similar to thatof the resist mask 141.

Further, after the source electrode 110 and the drain electrode 112 areformed, the oxide semiconductor film 108 (more specifically, the backchannel side thereof) is preferably cleaned. The oxide semiconductorfilm 108 is effectively cleaned by oxygen plasma treatment, cleaningtreatment by treatment with dilute hydrofluoric acid, or the like. Byperforming such a cleaning, an etching gas component used in forming thesource electrode 110 and the drain electrode 112, a residue of theresist mask 142, or the like can be removed from the oxide semiconductorfilm 108, so that the oxide semiconductor film 108 can be more purified.

Further, after the source electrode 110 and the drain electrode 112 areformed, heart treatment may be performed. The heat treatment isperformed at a temperature higher than or equal to 250° C. and lowerthan or equal to 650° C., preferably higher than or equal to 450° C. andlower than or equal to 600° C., or lower than the strain point of thesubstrate.

Through the above process, the transistor 150 described in thisembodiment is formed.

Next, the first insulating film 114 a is formed over the transistor 150,more specifically over the oxide semiconductor film 108, the sourceelectrode 110, and the drain electrode 112. Then, oxygen 145 isintroduced into the first insulating film 114 a and the oxidesemiconductor film 108 (see FIG. 4C).

The first insulating film 114 a can be formed using an oxide insulatingfilm such as a silicon oxide film, a gallium oxide film, an aluminumoxide film, a silicon oxynitride film, or an aluminum oxynitride film bya plasma CVD method or a sputtering method. The thickness of the firstinsulating film 114 a is preferably greater than or equal to 50 nm andless than or equal to 100 nm.

Further, the first insulating film 114 a is preferably an oxygen-excessoxide insulating film. With the oxygen-excess oxide insulating film,oxygen can be efficiently supplied to the oxide semiconductor film 108.

In this embodiment, a 30-nm-thick silicon oxynitride film is formed asthe first insulating film 114 a by a plasma CVD method. The conditionsfor forming the first insulating film 114 a can be as follows: forexample, the gas flow rate ratio of SiH₄ to N₂O is 20 sccm:3000 sccm;the pressure is 200 Pa; the RF power supply (power supply output) is 100W; and the substrate temperature is 350° C.±15° C. Note that like thegate insulating film 106, the first insulating film 114 a preferablycontains impurities such as water or hydrogen as little as possiblebecause it is an insulating film in contact with oxide semiconductorfilm 108.

The oxygen 145 contains at least any of an oxygen radical, ozone, anoxygen atom, and an oxygen ion (an oxygen molecular ion and/or an oxygencluster ion).

Introducing the oxygen 145 into the first insulating film 114 a can beperformed by, for example, an ion implantation method, an ion dopingmethod, a plasma immersion ion implantation method, plasma treatment, orthe like. Note that for the ion implantation method, a gas cluster ionbeam may be used. The oxygen 145 may be introduced into the entire areaof the first insulating film 114 a at a time. Alternatively, a linearion beam is used for introducing the oxygen 145. In the case of usingthe linear ion beam, the substrate or the ion beam is relatively moved(scanned), whereby the oxygen 145 can be introduced into the entire areaof the first insulating film 114 a.

As a gas for supplying the oxygen 145, a gas containing oxygen may beused. For example, an O₂ gas, an N₂O gas, a CO₂ gas, a CO gas, a NO₂gas, or the like can be used. Note that a rare gas (e.g., argon) may becontained in a gas for supplying oxygen.

Further, in the case where an ion implantation method is used forintroducing oxygen, the dose of the oxygen 145 is preferably greaterthan or equal to 1×10¹³ ions/cm² and less than or equal to 5×10¹⁶ions/cm². The content of oxygen in the first insulating film 114 a afterthe oxygen-introducing treatment preferably exceeds that of thestoichiometric composition of the first insulating film 114 a. The depthat which oxygen is implanted may be adjusted as appropriate byimplantation conditions.

Note that in the case where an oxide insulating film (e.g., a siliconoxide film or a silicon oxynitride film) is used as the first insulatingfilm 114 a, oxygen is one of main components in the oxide insulatingfilm. Therefore, it is difficult to estimate the oxygen concentration ofthe oxide insulating film accurately with secondary ion massspectrometry (SIMS) or the like. That is, it is difficult to judgewhether oxygen is intentionally added to the oxide insulating film ornot. Further, the same can be applied to the case where oxygen containedexcessively in the first insulating film 114 a is supplied to the oxidesemiconductor film 108 in a later step.

It is known that there are isotopes of oxygen, such as ¹⁷O and ¹⁸O, andthat the proportions of ¹⁷O and ¹⁸O in all of the oxygen atoms in natureare approximately 0.038% and approximately 0.2%, respectively. That isto say, it is possible to measure the concentrations of these isotopesin the oxide semiconductor film or the insulating film (the firstinsulating film 114 a in this embodiment) in contact with the oxidesemiconductor film by a method such as SIMS; therefore, the oxygenconcentration of the oxide semiconductor film or the insulating film incontact with the oxide semiconductor film may be able to be estimatedmore accurately by measuring the concentrations of these isotopes. Thus,the concentration of the isotope may be measured to determine whether ornot oxygen is intentionally added to the insulating film in contact withthe oxide semiconductor film.

By such a treatment for introducing the oxygen 145, the oxygen-excessfirst insulating film 114 a is formed. With the oxygen-excess firstinsulating film 114 a, oxygen can be supplied to the oxide semiconductorfilm 108 by solid-phase diffusion due to heat treatment performed in themanufacturing process of the transistor. By the treatment forintroducing the oxygen 145, oxygen may be introduced into the oxidesemiconductor film 108 through the first insulating film 114 a.

Next, the second insulating film 114 b is formed over the firstinsulating film 114 a (see FIG. 4D).

The second insulating film 114 b can be formed by a plasma CVD method ora sputtering method, using a silicon oxide film, a gallium oxide film,an aluminum oxide film, a silicon nitride film, a silicon oxynitridefilm, an aluminum oxynitride film, or a silicon nitride oxide film. Thethickness of the second insulating film 114 b is preferably greater thanor equal to 50 nm and less than or equal to 500 nm.

In this embodiment, as the second insulating film 114 b, a 370-nm-thicksilicon oxynitride film is formed by a plasma CVD method. The conditionsfor forming the second insulating film 114 b can be as follows: forexample, the gas flow rate ratio of SiH₄ to N₂O is 30 sccm:4000 sccm;the pressure is 200 Pa; the RF power supply (power supply output) is 150W; and the substrate temperature is 220° C.±15° C.

In the case where the first insulating film 114 a and the secondinsulating film 114 b are formed using the same kind of material, theinterface between the first insulating film 114 a and the secondinsulating film 114 b cannot be clearly defined in some cases.Accordingly, in this embodiment, the interface between the firstinsulating film 114 a and the second insulating film 114 b is shown by adotted line.

Note that like the first insulating film 114 a, the second insulatingfilm 114 b preferably contains impurities such as water or hydrogen aslittle as possible. Thus, in this embodiment, after formation of thesecond insulating film 114 b, heat treatment for removing hydrogen atoms(for dehydration or dehydrogenation) is performed thereon.

The temperature of heat treatment can be, for example, higher than orequal to 250° C. and lower than or equal to 600° C., preferably higherthan or equal to 300° C. and lower than or equal to 600° C. In thisembodiment, heat treatment is performed at 350° C. for one hour.

Next, an aluminum film 115 is formed over the second insulating film 114b (see FIG. 5A).

The aluminum film 115 is preferably formed by a sputtering method, anevaporation method, a CVD method, or the like. The thickness of thealuminum film 115 is preferably greater than or equal to 3 nm and lessthan or equal to 10 nm. In this embodiment, a 5-nm-thick aluminum filmis formed by a sputtering method.

The aluminum film 115 formed over the second insulating film 114 bbecomes an aluminum oxide film by being subjected to oxygen-introducingtreatment in a later step, and functions as a barrier film in thetransistor. The aluminum oxide film has a high shielding effect(blocking effect) of preventing penetration of both oxygen andimpurities such as hydrogen or water into the transistor, i.e., hasbarrier properties.

Next, oxygen 147 is introduced into the aluminum film 115. Thus, thealuminum film 115 becomes the aluminum oxide film 116 (see FIG. 5B).

The oxygen 147 can be introduced in a manner similar to that of theoxygen 145.

Further, by the treatment for introducing the oxygen 147, oxygen may beintroduced into part of the second insulating film 114 b through thealuminum film 115. Thus, in the second insulating film 114 b, oxygen canbe contained to compensate for oxygen which has been possibly releasedby the above heat treatment, and a region containing oxygen in excess ofthe stoichiometric composition can be formed. Note that such a regioncontaining oxygen in excess of the stoichiometric composition exists inat least part of the second insulating film 114 b. The depth at whichoxygen is implanted may be adjusted as appropriate by implantationconditions.

Further, in the aluminum oxide film 116, a region containing oxygen inexcess of the stoichiometric composition may be formed. Note that thealuminum oxide film 116 formed by the oxygen-introducing treatment doesnot need to contain oxygen equivalent to the stoichiometric compositionother than the region and may have some conductivity. For example, inthe case where the composition of the aluminum oxide film is representedby Al₂O_(x), x is preferably greater than or equal to 1 and less than orequal to 3.5. Further, in the case where the aluminum oxide film 116 hasconductivity, the resistivity ρ is preferably greater than or equal to10¹⁰ Ω19 m and less than or equal to 10¹⁹ Q·m, further preferablygreater than or equal to 10¹⁰ Ω·m and less than or equal to 10¹⁸ Q·m,still further preferably greater than or equal to 10¹¹ Ω·m and less thanor equal to 10¹⁵ Q·m. When the aluminum oxide film 116 has resistivityin the above range, the transistor 150 can be prevented from beingdamaged by electrostatic discharge.

In addition, the aluminum oxide film 116 is a film obtained by oxidizingthe aluminum film 115. Formation of the aluminum oxide film 116 byoxidation of the aluminum film 115 can increase productivity as comparedwith the case where an aluminum oxide film is deposited by a sputteringmethod.

Note that after introducing the oxygen 147 into the aluminum film 115,heat treatment may be performed. By the heat treatment, oxygen containedin the first insulating film 114 a or the second insulating film 114 bis supplied to the oxide semiconductor film 108, whereby oxygenvacancies in the oxide semiconductor film 108 may be filled. Thetemperature of the heat treatment can be, for example, higher than orequal to 250° C. and lower than or equal to 600° C., preferably higherthan or equal to 300° C. and lower than or equal to 600° C. In thisembodiment, heat treatment is performed at 300° C. for one hour.

Next, the planarization insulating film 118 is formed over the aluminumoxide film 116 (see FIG. 5C).

The planarization insulating film 118 can planarize the unevenness ofthe transistor 150. For the planarization insulating film 118, forexample, a heat-resistant organic material such as a polyimide-basedresin, an acrylic-based resin, a polyimide amide-based resin, abenzocyclobutene-based resin, a polyamide-based resin, or an epoxy-basedresin can be used. Other than such organic materials, a low-dielectricconstant material (a low-k material), a siloxane-based resin, or thelike can be used. Note that the planarization insulating film 118 may beformed by stacking a plurality of insulating films formed using any ofthese materials. In this embodiment, an acrylic-based resin film with athickness of 1.5 μm is used as the planarization insulating film 118.

As described above, in the transistor 150 in this embodiment, the oxidesemiconductor film is used for the channel formation region, and copperwhich is a low resistance material is used for the gate electrode, thesource electrode, and the drain electrode. Further, when the sourceelectrode and the drain electrode are formed, the back channel side ofthe oxide semiconductor film is not in contact with the copper film;thus, it is possible to prevent the copper element from being attachedor diffused in the oxide semiconductor film. In addition, the gateelectrode, the source electrode, and the drain electrode each use acopper film and have a barrier metal which can prevent diffusion of thecopper element. Therefore, a transistor with stable electriccharacteristics and little signal delay due to wiring resistance can beprovided.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 2

In this embodiment, a modification example of the semiconductor devicein Embodiment 1 and a method for manufacturing a semiconductor device,which is different from that in Embodiment 1, will be described withreference to FIGS. 6A and 6B, FIGS. 7A to 7D, and FIGS. 8A to 8D. Notethat portions similar to those in FIGS. 1A to 1C, FIGS. 2A to 2E, FIGS.3A to 3D, FIGS. 4A to 4D, and FIGS. 5A to 5C are denoted by the samereference numerals, and description thereof is omitted.

Structural Example 2 of Semiconductor Device

FIGS. 6A and 6B illustrate a structural example of a transistor 250 anda signal line region 260. FIG. 6A is a plan view of the transistor 250and the signal line region 260. FIG. 6B is a cross-sectional view takenalong the line X2-Y2 in FIG. 6A. Note that in FIG. 6A, some componentsof the transistor 250 and the signal line region 260 (e.g., a gateinsulating film 206 and a second metal film 210 b) are omitted to avoidcomplexity.

A semiconductor device illustrated in FIGS. 6A and 6B includes a gateelectrode 204 formed over the substrate 102, a gate insulating film 206formed over the gate electrode 204, the oxide semiconductor film 108formed in contact with the gate insulating film 206 and in a positionwhich overlaps with the gate electrode 204, a source electrode 210 and adrain electrode 212 formed over the oxide semiconductor film 108, and asignal line 232 electrically connected to the source electrode 210. Thesignal line 232 includes a first metal film 210 a, a second metal film210 b and a third metal film 210 c. The second metal film 210 b isformed in the inner region of the end portions of the first metal film210 a and the third metal film 210 c. The source electrode 210 includesthe first metal film 210 a and the third metal film 210 c. The drainelectrode 212 includes the first metal film 212 a and the third metalfilm 212 c.

The gate electrode 204 includes a first gate electrode 204 a and asecond gate electrode 204 b. As the first gate electrode 204 a, a metalfilm or a metal nitride film containing one or more elements selectedfrom tungsten, tantalum, titanium, and molybdenum is preferably used.The second gate electrode 204 b preferably contains a copper element.For example, in this embodiment, a tungsten film is used as the firstgate electrode 204 a and a copper film is used as the second gateelectrode 204 b. With such a stacked-layer structure, the gate electrode204 can have low resistance. By providing the first gate electrode 204a, adhesion between the substrate 102 and the copper film used as thesecond gate electrode 204 b can be improved and/or diffusion of thecopper element in the copper film used as the second gate electrode 204b can be prevented.

The gate insulating film 206 includes a first gate insulating film 206 aand a second gate insulating film 206 b. The first gate insulating film206 a may at least have a function of preventing diffusion of the copperelement in the copper film used as the second gate electrode 204 b. Asthe first gate insulating film 206 a, a silicon nitride film, a siliconnitride oxide film, an aluminum oxide film, an aluminum nitride oxidefilm, or the like can be used. The second gate insulating film 206 b mayat least have a function of supplying oxygen to the oxide semiconductorfilm 108 to be formed later. As the second gate insulating film 206 b, asilicon oxide film, a silicon oxynitride film, or the like can be used.For example, in this embodiment, a silicon nitride film is used as thefirst gate insulating film 206 a, and a silicon oxynitride film is usedas the second gate insulating film 206 b. With the gate insulating film206 having such a stacked-layer structure, diffusion of the copperelement in the copper film used as the gate electrode 204 can beprevented and oxygen can be supplied to the oxide semiconductor film 108to be formed later.

As the first metal film 210 a, the first metal film 212 a, the thirdmetal film 210 c, and the third metal film 212 c, a metal film or ametal nitride film containing one or more elements selected fromtungsten, tantalum, titanium, and molybdenum is preferably used.

For example, in this embodiment, a tungsten film is used as the firstmetal film 210 a and the first metal film 212 a and a tantalum nitridefilm is used as the third metal film 210 c and the third metal film 212c.

The second metal film 210 b preferably contains a copper element. Inthis embodiment, a copper film is used as the second metal film 210 b.

As described above, the structures of the source electrode 210 and thedrain electrode 212 used for the transistor 250 are different from thestructure of the signal line 232. By electrically connecting the sourceelectrode 210 and the drain electrode 212 to the signal line 232including a copper film, signal delay and the like due to wiringresistance can be suppressed. Moreover, with the structure where amaterial including a copper element is not used in the source electrode210 and the drain electrode 212 used for the transistor 250, the copperelement which might be diffused into the oxide semiconductor film 108can be disposed away from the oxide semiconductor film 108, which iseffective. Furthermore, the signal line 232, the source electrode 210,and the drain electrode 212 can be manufactured in the same steps of asemiconductor manufacturing process; thus, an excellent effect ofreducing manufacturing cost is obtained.

Next, a method for manufacturing the transistor 250 and the signal lineregion 260 illustrated in FIGS. 6A to 6B will be described withreference to FIGS. 7A to 7D and FIGS. 8A to 8D.

Method 2 for Manufacturing Semiconductor Device

First, the gate electrode 204, the gate insulating film 206, and theoxide semiconductor film 108 are formed over the substrate 102. Notethat referring to the process of FIGS. 2A to 2D in Embodiment 1, thegate electrode 204, the gate insulating film 206, and the oxidesemiconductor film 108 can be formed. Then, a first metal film 209 a anda second metal film 209 b that are to serve as the source electrode, thedrain electrode, and the signal line are formed over the gate insulatingfilm 206 and the oxide semiconductor film 108 (see FIG. 7A).

The first metal film 209 a is preferably a metal film or a metal nitridefilm containing one or more elements selected from tungsten, tantalum,titanium, and molybdenum. In this embodiment, as the first metal film209 a, a 50-nm-thick tungsten film formed by a sputtering method isused.

Further, the first metal film 209 a may have a stacked-layer structure.For example, the stacked-layer structure of the first metal film 209 aincludes a metal film containing one or more elements selected fromtungsten, tantalum, titanium, and molybdenum as a first layer, and ametal nitride film containing one or more elements selected fromtungsten nitride, tantalum nitride, titanium nitride and molybdenumnitride as a second layer.

For the first metal film 209 a, since the first metal film 209 a is incontact with the oxide semiconductor film 108, a material which does notextract oxygen from the oxide semiconductor film 108 and does not makethe oxide semiconductor film 108 have n-type conductivity, or a materialwhich is not diffused into the oxide semiconductor film 108 and does notmake the oxide semiconductor film 108 have n-type conductivity is used.In addition, a material which prevents diffusion of a copper elementfrom a copper film used as the second metal film 209 b to the oxidesemiconductor film 108 is preferably used for the first metal film 209a.

The second metal film 209 b preferably contains a copper element. Notethat a copper alloy or the like in which aluminum, gold, silver, zinc,tin, nickel or the like is added at several weight percent to copper maybe used. In this embodiment, as the second metal film 209 b, a200-nm-thick copper film formed by a sputtering method is used.

Next, a resist is applied over the second metal film 209 b, and thefirst patterning is performed, whereby a resist mask 241 is formed (seeFIG. 7B).

The resist mask 241 can be formed using a material and a method similarto those of the resist mask 141 in Embodiment 1.

Next, the second metal film 209 b is partly removed by the firstetching, whereby the second metal film 210 b is formed (see FIG. 7C).

As a method for removing the second metal film 209 b, a wet etchingmethod is preferable. As a chemical solution used in the wet etchingmethod, it is preferable that a chemical solution which can etch thesecond metal film 209 b and does not remove the first metal film 209 abe used. For example, in the case where a tungsten film is used as thefirst metal film 209 a and a copper film is used as the second metalfilm 209 b, a mixed solution of water, hydrogen peroxide water, andcarboxylic acid; a mixed solution of water, phosphoric acid, nitricacid, sulfuric acid, and potassium sulfate; or the like can be used as achemical solution.

Moreover, isotropic etching may be performed by adjusting a wet etchingtime so that the side surfaces of the second metal film 210 b may bereceded to be at the inner side of the side surfaces of the resist mask241.

In this manner, when the first etching is performed, the second metalfilm 209 b in the signal line region 260 is left and the second metalfilm 209 b is removed from a region where the oxide semiconductor film108 is formed.

Next, the resist mask 241 is removed, whereby the third metal film 209 cis formed over the first metal film 209 a and the second metal film 210b (see FIG. 7D).

Note that the resist mask 241 can be removed by a method similar to thatof the resist mask 141 in Embodiment 1.

The third metal film 209 c can be formed using a method and a materialsimilar to those of the first metal film 209 a. Note that in thisembodiment, as the third metal film 209 c, a 100-nm-thick tantalumnitride film formed by a sputtering method is used.

Next, a resist is applied over the third metal film 209 c, and thesecond patterning is performed, whereby a resist mask 242 is formed (seeFIG. 8A).

The resist mask 242 can be formed using a material and a method similarto those of the resist mask 241.

Next, the first metal film 209 a and the third metal film 209 c ispartly removed by the second etching, whereby the first metal film 210a, the first metal film 212 a, the third metal film 210 c, and the thirdmetal film 212 c are formed (see FIG. 8B).

Note that the first metal film 209 a and the third metal film 209 c arepartly removed by the second etching so that the removed portions are atthe outer side of the end portions of the second metal film 210 b whichis removed by the first etching.

As a method for removing the first metal film 209 a and the third metalfilm 209 c, a dry etching method is preferably used. As a gas used for adry etching method, in the case where a tungsten film is used as thefirst metal film 209 a and a tantalum nitride film is used as the thirdmetal film 209 c, a mixed gas containing SF₆ and O₂, a mixed gascontaining SF₆ and BCl₃, or the like can be used, for example.

Note that it is desirable that etching conditions be optimized so as notto etch and divide the oxide semiconductor film 108 when the first metalfilm 209 a and the third metal film 209 c are etched. However, it isdifficult to obtain etching conditions in which only the first metalfilm 209 a and the third metal film 209 c are etched and the oxidesemiconductor film 108 is not etched at all. In some cases, when thefirst metal film 209 a and the third metal film 209 c are etched, theoxide semiconductor film 108 is partly etched to have a groove portion(a recessed portion).

Next, the resist mask 242 is removed, whereby the source electrode 210including the first metal film 210 a and the third metal film 210 c, andthe drain electrode 212 including the first metal film 212 a and thethird metal film 212 c are formed. Further, in the signal line region260, the signal line 232 including the first metal film 210 a, thesecond metal film 210 b, and the third metal film 210 c is formed (seeFIG. 8C).

In this manner, the signal line 232 which uses a copper film as thesecond metal film 210 b, and the source electrode 210 and the drainelectrode 212 which do not include the second metal film 210 b can beformed in the same steps.

Note that the resist mask 242 can be removed by a method similar to thatof the resist mask 241.

Further, after the signal line 232, the source electrode 210, and thedrain electrode 212 are formed, the oxide semiconductor film 108 (morespecifically, the back channel side thereof) is preferably cleaned. Theoxide semiconductor film 108 is effectively cleaned by oxygen plasmatreatment, cleaning treatment by treatment with dilute hydrofluoricacid, or the like. By performing such a cleaning, an etching gascomponent used in forming the source electrode 210 and the drainelectrode 212, a residue of the resist mask 242, or the like can beremoved from the oxide semiconductor film 108, so that the oxidesemiconductor film 108 can be more purified.

Further, after the signal line 232, the source electrode 210, and thedrain electrode 212 are formed, heart treatment may be performed. Theheat treatment is performed at a temperature higher than or equal to250° C. and lower than or equal to 650° C., preferably higher than orequal to 450° C. and lower than or equal to 600° C., or lower than thestrain point of the substrate.

Through the above process, the transistor 250 and the signal line region260 in this embodiment are formed.

Next, the first insulating film 114 a, the second insulating film 114 b,the aluminum oxide film 116, and the planarization insulating film 118are formed over the transistor 250 and the signal line region 260 (seeFIG. 8D).

Referring to the process in Embodiment 1, the first insulating film 114a, the second insulating film 114 b, the aluminum oxide film 116, andthe planarization insulating film 118 can be formed.

As described above, the structures of the source electrode 210 and thedrain electrode 212 of the transistor 250 are different from thestructure of the signal line 232 of the signal line region 260. Byelectrically connecting the source electrode 210 and the drain electrode212 to the signal line 232 using the copper film, signal delay and thelike due to wiring resistance can be suppressed. Moreover, with thestructure where a material including a copper element is not used in thesource electrode 210 and the drain electrode 212 used for the transistor250, the copper element which might be diffused into the oxidesemiconductor film 108 can be disposed away from the oxide semiconductorfilm 108, which is effective. Furthermore, the signal line 232, thesource electrode 210 and the drain electrode 212 can be manufactured inthe same steps of a semiconductor manufacturing process; thus, anexcellent effect of reducing manufacturing cost is obtained.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 3

A display device with a display function can be manufactured using thetransistor exemplified in Embodiment 1 or Embodiment 2 and the signalline exemplified in Embodiment 2. Moreover, some or all of the drivercircuits which include the transistor can be formed over a substratewhere the pixel portion is formed, whereby a system-on-panel can beobtained. An example of the display device will be described withreference to FIG. 9.

In FIG. 9, a sealant 312 is provided so as to surround a pixel portion302, a source driver circuit portion 304, and a gate driver circuitportion 306 which are provided over a first substrate 300. The secondsubstrate 301 is provided over the pixel portion 302, the source drivercircuit portion 304, and the gate driver circuit portion 306. Thus, thepixel portion 302, the source driver circuit portion 304, and the gatedriver circuit portion 306 are sealed together with a display element bythe first substrate 300, the sealant 312, and the second substrate 301.

In FIG. 9, a flexible printed circuit (FPC) terminal portion 308 whichis electrically connected to the pixel portion 302, the source drivercircuit portion 304, and the gate driver circuit portion 306 is providedin a region over the first substrate 300 that is different from theregion surrounded by the sealant 312. An FPC 316 is connected to the FPCterminal portion 308. Signals and potentials applied to the pixelportion 302, the source driver circuit portion 304, and the gate drivercircuit portion 306 are supplied through the FPC 316.

Further, in FIG. 9, a signal line 310 is connected to the pixel portion302, the source driver circuit portion 304, the gate driver circuitportion 306, and FPC terminal portion 308. Signals and potentials areapplied to the pixel portion 302, the source driver circuit portion 304,the gate driver circuit portion 306, and the FPC terminal portion 308via the signal line 310 from the FPC 316.

In FIG. 9, an example in which the source driver circuit portion 304 andthe gate driver circuit portion 306 are formed over the first substrate300 where the pixel portion 302 is also formed is described; however,the structure is not limited thereto. For example, only the gate drivercircuit portion 306 may be formed over the first substrate 300 or onlythe source driver circuit portion 304 may be formed over the firstsubstrate 300. In this case, a substrate which is separately preparedand where a source driver circuit, a gate driver circuit, or the like isformed (e.g., a driver-circuit substrate formed using a single-crystalsemiconductor film or a polycrystalline semiconductor film) may bemounted on the first substrate 300.

The connection method of such a separately formed driver circuitsubstrate is not particularly limited; for example, a chip on glass(COG) method, a wire bonding method, or a tape automated bonding (TAB)method can be used.

The display device includes, in its category, a panel in which a displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device includes any of the followingmodules in its category: a module including a connector such as an FPC,a TAB tape, or tape carrier package (TCP); a module including a TAB tapeor a TCP which is provided with a printed wiring board at the endthereof; and a module including a driver circuit substrate or an ICwhich is directly mounted on a display element by a COG method. Inaddition, the pixel portion 302, the source driver circuit portion 304,and the gate driver circuit portion 306 provided over the firstsubstrate 300 each include a plurality of transistors. The plurality oftransistors can be the transistors exemplified in Embodiments 1 and 2.In this embodiment, the case where the transistor exemplified inEmbodiment 2 is applied will be described.

In addition, as a display element provided in the display device, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. The light-emitting elementincludes, in its category, an element whose luminance is controlled by acurrent or a voltage, and specifically includes, in its category, aninorganic electroluminescent (EL) element, an organic EL element, andthe like. Furthermore, a display medium whose contrast is changed by anelectric effect, such as electronic ink, can be used.

Embodiments of the display elements provided in display devices aredescribed with reference to FIG. 10 and FIG. 11. FIG. 10 and FIG. 11each correspond to a cross-sectional view of a display device takenalong the dotted line Q-R in FIG. 9.

In the display device illustrated in FIG. 10, the FPC terminal portion308 provided over the first substrate 300 includes a terminal electrode360 including a first metal film 360 a, a second metal film 360 b, and athird metal film 360 c. The terminal electrode 360 is electricallyconnected to a terminal of the FPC 316 via an anisotropic conductivefilm 380.

The terminal electrode 360 is formed through the same steps as sourceelectrodes and drain electrodes of a transistor 350 and a transistor 352and the signal line 310.

Further, the pixel portion 302 and the source driver circuit portion 304over the first substrate 300 include a plurality of transistors. As theplurality of transistors, the transistor 350 included in the pixelportion 302 and the transistor 352 included in the source driver circuitportion 304 are exemplified in FIG. 10 and FIG. 11.

Note that in this embodiment, the transistor 350 included in the pixelportion 302 and the transistor 352 included in the source driver circuitportion 304 have in the same size; however, this embodiment is notlimited to this. The transistor used in the pixel portion 302 and thetransistor used in the source driver circuit portion 304 can beappropriately changed in size (L/W) or in the transistor count. The gatedriver circuit portion 306 is not illustrated in FIG. 10 and FIG. 11;however, the gate driver circuit portion 306 can have a structuresimilar to that of the source driver circuit portion 304. Note that theportion to which the gate driver circuit portion 306 is connected, theconnecting method, and the like are different from those of the sourcedriver circuit portion 304.

Further, the transistor 350, the transistor 352, and the signal line 310in FIG. 10 and FIG. 11 can have the structures similar to those of thetransistor 250 and the signal line 232 in Embodiment 2.

That is, the transistor 350 and the transistor 352 each have the sourceelectrode and the drain electrode including the first metal film and thethird metal film, and the signal line 310 has a wiring including thefirst metal film, the second metal film, and the third metal film. Thefirst metal film and the third metal film are a metal film or a metalnitride film containing one or more elements selected from tungsten,tantalum, titanium, and molybdenum. The second metal film is formedusing a material containing a copper element.

Moreover, the terminal electrode 360 has a structure similar to that ofthe signal line 310 and is formed using the first metal film, the secondmetal film, and the third metal film.

As described above, the source electrodes and the drain electrodes inthe transistor 350 and the transistor 352 are formed without using acopper film, and the signal line 310 and the terminal electrode 360 areformed using a copper film. By using the transistor 350, the transistor352, the signal line 310, and the terminal electrode 360, a displaydevice having stable electric characteristics and includinglow-resistance electrodes and wirings can be provided.

Further, in FIG. 10 and FIG. 11, an insulating film 364, a protectiveinsulating film 366, and a planarization insulating film 368 areprovided over the transistor 350 and the transistor 352.

In this embodiment, a silicon oxynitride film is used as the insulatingfilm 364, and an aluminum oxide film is used as the protectiveinsulating film 366. Note that the insulating film 364 and theprotective insulating film 366 can be formed by a sputtering method or aplasma CVD method.

The silicon oxynitride film provided as the insulating film 364 isprovided in contact with the oxide semiconductor film and can supplyoxygen to the oxide semiconductor film.

The aluminum oxide film provided as the protective insulating film 366has a high shielding effect (blocking effect) of preventing penetrationof both oxygen and impurities such as hydrogen and water. Therefore, inand after the manufacturing process, the aluminum oxide film functionsas a protective film for preventing entry of an impurity such ashydrogen or water, which causes a change, into the oxide semiconductorfilm and release of oxygen, which is a main constituent material of theoxide semiconductor film, from the oxide semiconductor film.

Further, the planarization insulating film 368 can be formed using anorganic material having heat resistance, such as a polyimide-basedresin, an acrylic-based resin, a polyimide amide-based resin, abenzocyclobutene-based resin, a polyamide-based resin, or an epoxy-basedresin. Note that the planarization insulating film 368 may be formed bystacking a plurality of insulating films formed using these materials.

Further, the display device in this embodiment has a structure in whichthe planarization insulating film 368 is provided over the transistor352 included in the source driver circuit portion 304 and the conductivefilm 370 a is provided over the planarization insulating film 368 tooverlap with a channel formation region of the oxide semiconductor film.However, without limitation to this structure, the conductive film 370 ais not necessarily provided. By providing the conductive film 370 a soas to overlap with the channel formation region of the oxidesemiconductor film, the amount of change in the threshold voltage of thetransistor 352 between before and after the BT test can be reduced.Potential of the conductive film 370 a may be the same as or differentfrom that of a gate electrode of the transistor 352. The conductive film370 a can also function as a second gate electrode. The potential of theconductive film 370 a may be GND or 0 V, or the conductive film 370 amay be in a floating state.

Note that the conductive film 370 a has a function of blocking anexternal electric field (particularly, to block static electricity),that is, to prevent an external electric field from acting on the inside(a circuit portion including the transistor 352). Such a blockingfunction of the conductive film 370 a can prevent variation in electriccharacteristics of the transistor 352 due to the influence of anexternal electric field such as static electricity. Note that theconductive film 370 a may be formed in a wide range of area so as tooverlap with the transistor 352. Accordingly, the function of blockingstatic electricity is further improved.

The display device in this embodiment has a structure in which theplanarization insulating film 368 is provided over the transistor 350included in the pixel portion 302 and the conductive film 370 b incontact with the source electrode or the drain electrode is providedover the planarization insulating film 368. The conductive film 370 bserves as a pixel electrode in the pixel portion 302.

The transistor 350 included in the pixel portion 302 is electricallyconnected to a display element to form a display panel. A variety ofdisplay elements can be used as the display element as long as displaycan be performed.

An example of a liquid crystal display device using a liquid crystalelement as a display element is illustrated in FIG. 10. In FIG. 10, aliquid crystal element 402 which is a display element includes theconductive film 370 b, a counter electrode 404, and a liquid crystallayer 406. An insulating film 410 and an insulating film 412 serving asalignment films are provided so that the liquid crystal layer 406 isinterposed therebetween. The counter electrode 404 is provided on thesecond substrate 301 side and stacked over the conductive film 370 bwith the liquid crystal layer 406 provided therebetween.

A spacer 435 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the thickness (acell gap) of the liquid crystal layer 406. Alternatively, a sphericalspacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric fieldmode, a liquid crystal exhibiting a blue phase for which an alignmentfilm is unnecessary may be used. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition which includes a liquid crystal showing a blue phase and achiral agent has a short response time and has optical isotropy, whichmakes the alignment process unneeded and the viewing angle dependencesmall. In addition, since an alignment film does not need to be providedand rubbing treatment is unnecessary, electrostatic discharge damagecaused by the rubbing treatment can be prevented and defects and damageof the liquid crystal display device can be reduced in the manufacturingprocess. Thus, productivity of the liquid crystal display device can beincreased. A transistor including an oxide semiconductor film has apossibility that the electric characteristics of the transistor may varysignificantly by the influence of static electricity and deviate fromthe designed range. Therefore, it is more effective to use a liquidcrystal material exhibiting a blue phase for the liquid crystal displaydevice including the transistor that includes the oxide semiconductorfilm.

The specific resistivity of the liquid crystal material is higher thanor equal to 1×10⁹ Q·cm, preferably higher than or equal to 1×10¹¹ Q·cm,further preferably higher than or equal to 1×10¹² Q·cm. Note that thespecific resistance in this specification is measured at 20° C.

The size of storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. The size of the storage capacitor may be setconsidering the off-state current of a transistor or the like. By usinga transistor including an oxide semiconductor film which is highlypurified and in which formation of an oxygen vacancy is suppressed, itis enough to provide a storage capacitor having a capacitance that is ⅓or less, preferably ⅕ or less of liquid crystal capacitance of eachpixel.

In the transistor used in this embodiment, which includes an oxidesemiconductor film which is highly purified and in which formation of anoxygen vacancy is suppressed, the current in an off state (off-statecurrent) can be made small.

Accordingly, an electrical signal such as an image signal can be heldfor a longer period in the pixel, and a writing interval can be setlonger in an on state. Accordingly, frequency of refresh operation canbe reduced, which leads to an effect of suppressing power consumption.

The transistor used in this embodiment, which includes an oxidesemiconductor film which is highly purified and in which formation of anoxygen vacancy is suppressed, can have relatively high field-effectmobility and thus can operate at high speed. For example, with such atransistor which can operate at high speed used for a liquid crystaldisplay device, a switching transistor in a pixel portion and a drivertransistor in a driver circuit portion can be formed over one substrate.That is, since a semiconductor device formed of a silicon wafer or thelike is not additionally needed as a driver circuit, the number ofcomponents of the semiconductor device can be reduced. In addition, byusing a transistor which can operate at high speed in a pixel portion, ahigh-quality image can be provided.

Further, a wiring containing a copper element is used as a signal lineconnected to a switching transistor in a pixel portion and a drivertransistor in a driver circuit portion, which results in less signaldelay and the like due to wiring resistance, and can be used for adisplay device with large screen.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modeis preferable. Some examples are given as the vertical alignment mode.For example, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, and the like can be given. Furthermore,this embodiment can be applied to a VA liquid crystal display device.The VA liquid crystal display device has a kind of form in whichalignment of liquid crystal molecules of a liquid crystal display panelis controlled. In the VA liquid crystal display device, liquid crystalmolecules are aligned in a vertical direction with respect to a panelsurface when no voltage is applied. Moreover, it is possible to use amethod called domain multiplication or multi-domain design, in which apixel is divided into some regions (subpixels) and molecules are alignedin different directions in their respective regions.

In the display device, a black matrix (a light-blocking layer), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

As a display method in the pixel portion, a progressive method, aninterlace method or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite); R, G, B, and one or more of yellow, cyan, magenta, and the like;or the like can be used. Further, the sizes of display regions may bedifferent between respective dots of color elements. Note that thedisclosed invention is not limited to the application to a displaydevice for color display; the disclosed invention can also be applied toa display device for monochrome display.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element as a light-emitting element is described here.

In order to extract light emitted from the light-emitting element, it isacceptable as long as at least one of a pair of electrodes has alight-transmitting property. A transistor and a light-emitting elementare formed over a substrate. The light-emitting element can have a topemission structure in which light emission is extracted through thesurface opposite to the substrate; a bottom emission structure in whichlight emission is extracted through the surface on the substrate side;or a dual emission structure in which light emission is extractedthrough the surface opposite to the substrate and the surface on thesubstrate side, and a light-emitting element having any of theseemission structures can be used.

An example of a display device in which a light-emitting element is usedas a display element is illustrated in FIG. 11. A light-emitting element450 which is a display element is electrically connected to thetransistor 350 provided in the pixel portion 302. Note that thestructure of the light-emitting element 450 is a stacked structure ofthe conductive film 370 b, an electroluminescent layer 452, and an upperelectrode 454, however, the structure is not limited thereto. Thestructure of the light-emitting element 450 can be changed asappropriate depending on the direction in which light is extracted fromthe light-emitting element 450, or the like.

A partition wall 456 is made of an organic insulating material or aninorganic insulating material. It is particularly preferable that thepartition wall 456 is formed using a photosensitive resin material. Forexample, in the case where the partition wall 456 is formed using aphotosensitive resin material, by applying the photosensitive resinmaterial over the planarization insulating film 368 and the conductivefilm 370 b and irradiating a desired area thereof with light, an openingportion whose sidewall is formed as an inclined surface with continuouscurvature is formed over part of the conductive film 370 b.

The electroluminescent layer 452 may be formed using either a singlelayer or a plurality of layers stacked.

A protective film may be formed over the upper electrode 454 and thepartition wall 456 in order to prevent oxygen, hydrogen, water, carbondioxide, or the like from entering the light-emitting element 450. Asthe protective film, a silicon nitride film, a silicon nitride oxidefilm, or the like can be used. In addition, in a space which is formedwith the first substrate 300, the second substrate 301, and the sealant312, a filler 458 is provided and sealed. It is preferable that a panelbe packaged (sealed) with a protective film (such as a laminate film oran ultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the panel is not exposedto the outside air, in this manner.

As the filler 458, an ultraviolet curable resin or a thermosetting resincan be used as well as an inert gas such as nitrogen or argon. Forexample, polyvinyl chloride (PVC), an acrylic-based resin, apolyimide-based resin, an epoxy-based resin, a silicone-based resin,polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA) can be used.For example, nitrogen is used for the filler 458.

If necessary, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate for a light-emitting surfaceof the light-emitting element. Further, the polarizing plate or thecircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Note that in FIG. 10 and FIG. 11, a flexible substrate as well as aglass substrate can be used as the first substrate 300 and the secondsubstrate 301. For example, a light-transmitting plastic substrate orthe like can be used. As plastic, a fiberglass-reinforced plastics (FRP)plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylicresin film can be used. In addition, a sheet with a structure in whichan aluminum foil is sandwiched between PVF films or polyester films canbe used.

As described above, by using the transistors described in Embodiment 1or Embodiment 2 and the signal line in Embodiment 2, a display devicehaving a variety of functions can be provided.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 4

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofthe electronic devices are a television device (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, electronic paper, a camera such as a digital camera or a digitalvideo camera, a digital photo frame, a mobile phone (also referred to asa mobile telephone or a mobile phone device), a portable game console, aportable digital assistant (PDA), a portable terminal (a smart phone, atablet PC, and the like are included), an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.Examples of electronic devices each including the semiconductor devicedescribed in any of the above embodiments are described with referenceto FIGS. 12A to 12F and FIGS. 13A to 13D.

FIG. 12A illustrates a laptop personal computer which includes a mainbody 3001, a housing 3002, a display portion 3003, a keyboard 3004, andthe like. The semiconductor device described in any of the aboveembodiments is applied to the display portion 3003, whereby a laptoppersonal computer with stable electric characteristics and little signaldelay due to wiring resistance can be provided.

FIG. 12B is a portable digital assistant (PDA) including a displayportion 3023, an external interface 3025, an operation button 3024, andthe like in a main body 3021. A stylus 3022 is included as an accessoryfor operation. The semiconductor device described in any of the aboveembodiments is applied to the display portion 3023, whereby a portabledigital assistant (PDA) with stable electric characteristics and littlesignal delay due to wiring resistance can be provided.

FIG. 12C illustrates an example of an e-book reader. For example, thee-book reader 2700 includes two housings, a housing 2701 and a housing2703. The housing 2701 and the housing 2703 are combined with a hinge2711 so that the e-book reader 2700 can be opened and closed with thehinge 2711 as an axis. With such a structure, the e-book reader 2700 canoperate like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the structure where different images are displayed ondifferent display portions, for example, the right display portion (thedisplay portion 2705 in FIG. 12C) can display text and the left displayportion (the display portion 2707 in FIG. 12C) can display images. Thesemiconductor device described in any of the above embodiments isapplied to the display portion 2705 and the display portion 2707,whereby an e-book reader with stable electric characteristics and littlesignal delay due to wiring resistance can be provided. In the case ofusing a transflective or reflective liquid crystal display device as thedisplay portion 2705, the e-book reader may be used in a comparativelybright environment; therefore, a solar cell may be provided so thatpower generation by the solar cell and charge by a battery can beperformed. When a lithium ion battery is used as the battery, there areadvantages of downsizing and the like.

FIG. 12C illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, operation keys 2723, a speaker 2725,and the like. With the operation key 2723, pages can be turned. Notethat a keyboard, a pointing device, or the like may also be provided onthe surface of the housing, on which the display portion is provided.Furthermore, an external connection terminal (an earphone terminal, aUSB terminal, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Moreover, the e-book reader 2700 may have a function of anelectronic dictionary.

The e-book reader 2700 may have a configuration capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

FIG. 12D is a mobile phone including two housings, a housing 2800 and ahousing 2801. The housing 2801 includes a display panel 2802, a speaker2803, a microphone 2804, a pointing device 2806, a camera lens 2807, anexternal connection terminal 2808, and the like. In addition, thehousing 2800 includes a solar cell 2810 having a function of charge ofthe mobile phone, an external memory slot 2811, and the like. Further,an antenna is incorporated in the housing 2801. The semiconductor devicedescribed in any of the above embodiments is applied to the displaypanel 2802, a mobile phone with stable electric characteristics andlittle signal delay due to wiring resistance can be provided.

Further, the display panel 2802 is provided with a touch screen. Aplurality of operation keys 2805 which is displayed is indicated bydashed lines in FIG. 12D. Note that a boosting circuit by which avoltage output from the solar cell 2810 is increased to be sufficientlyhigh for each circuit is also included.

In the display panel 2802, the display direction can be appropriatelychanged depending on a usage pattern. Further, the display device isprovided with the camera lens 2807 on the same surface as the displaypanel 2802, and thus it can be used as a video phone. The speaker 2803and the microphone 2804 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Moreover, thehousings 2800 and 2801 in a state where they are developed asillustrated in FIG. 12D can shift by sliding so that one is lapped overthe other; therefore, the size of the mobile phone can be reduced, whichmakes the mobile phone suitable for being carried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Moreover, a largeamount of data can be stored by inserting a storage medium into theexternal memory slot 2811 and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 12E is a digital video camera including a main body 3051, a displayportion A 3057, an eyepiece 3053, an operation switch 3054, a displayportion B 3055, a battery 3056, and the like. The semiconductor devicedescribed in any of the above embodiments is applied to the displayportion A 3057 and the display portion B 3055, whereby a digital videocamera with stable electric characteristics and little signal delay dueto wiring resistance can be obtained.

FIG. 12F shows an example of a television set. In the television set9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605. The semiconductor device described in any ofthe above embodiments is applied to the display portion 9603, whereby atelevision set with stable electric characteristics and little signaldelay due to wiring resistance can be provided.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the display device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIGS. 13A to 13D illustrate examples of a tablet terminal. FIGS. 13A to13C illustrate a tablet terminal 5000. FIG. 13D illustrates a tabletterminal 6000.

FIGS. 13A to 13C are a front view, a side view, and a rear view of thetablet terminal 5000, respectively. FIG. 13D is a front view of thetablet terminal 6000.

The tablet terminal 5000 includes a housing 5001, a display portion5003, a power button 5005, a front camera 5007, a rear camera 5009, afirst external connection terminal 5011, a second external connectionterminal 5013, and the like.

In addition, the display portion 5003 is incorporated in the housing5001 and can be used as a touch panel. For example, e-mailing orschedule management can be performed by touching an icon 5015 and thelike on the display portion 5003. Further, the front camera 5007 isincorporated on the front side of the housing 5001, whereby an image onthe user's side can be taken. The rear camera 5009 is incorporated inthe rear side of the housing 5001, whereby an image on the opposite sideof the user can be taken. Further, the housing 5001 includes the firstexternal connection terminal 5011 and the second external connectionterminal 5013. Sound can be output to an earphone or the like throughthe first external connection terminal 5011, and data can be movedthrough the second external connection terminal 5013, for example.

The tablet terminal 6000 in FIG. 13D includes a first housing 6001, asecond housing 6003, a hinge portion 6005, a first display portion 6007,a second display portion 6009, a power button 6011, a first camera 6013,a second camera 6015, and the like.

The first display portion 6007 is incorporated in the first housing6001. The second display portion 6009 is incorporated in the secondhousing 6003. For example, the first display portion 6007 and the seconddisplay portion 6009 are used as a display panel and a touch panel,respectively. By looking at a text icon 6017 displayed on the firstdisplay portion 6007 by touching the icon 6019 or a keyboard 6021 (akeyboard image, actually) displayed on the second display portion 6009,image selecting, text input, and the like can be made. Alternatively,the first display portion 6007 and the second display portion 6009 maybe a touch panel and a display panel, respectively, or the first displayportion 6007 and the second display portion 6009 may be touch panels.

The first housing 6001 and the second housing 6003 are connected to eachother and open and close on the hinge portion 6005. With this structure,when the first display portion 6007 incorporated in the first housing6001 and the second display portion 6009 incorporated in the secondhousing 6003 are faced each other in carrying the tablet terminal 6000,the surfaces of the first display portion 6007 and the second displayportion 6009 (e.g., plastic substrates) can be protected, which ispreferable.

Alternatively, the first housing 6001 and the second housing 6003 may beseparated by the hinge portion 6005 (convertible type). Thus, theapplication range of the tablet terminal 6000 can be extended, and forexample, the first housing 6001 is used in a vertical orientation andthe second housing 6003 is used in a horizontal orientation.

Further, the first camera 6013 and the second camera 6015 can take 3Dimages.

The tablet terminal 5000 and the tablet terminal 6000 may send andreceive data wirelessly. For example, through wireless internetconnection, desired data can be purchased and downloaded.

The tablet terminals 5000 and 6000 can have other functions such as afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing the data displayed on the displayportion by touch input, and a function of controlling processing byvarious kinds of software (programs). A detector such as a photodetectorcapable of optimizing display luminance in accordance with the amount ofoutside light or a sensor for detecting inclination, like a gyroscope oran acceleration sensor, can be included.

The semiconductor device described in any of the above embodiments isapplied to the display portion 5003 of the tablet terminal 5000, thefirst display portion 6007 of the tablet terminal 6000, and/or thesecond display portion 6009, the tablet terminal with stable electriccharacteristics and little signal delay due to wiring resistance can beprovided.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

This application is based on Japanese Patent Application serial no.2012-026624 filed with Japan Patent Office on Feb. 9, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising the steps of: forming a gate electrode; forming agate insulating film over the gate electrode; forming an oxidesemiconductor film over the gate electrode with the gate insulating filminterposed between the gate electrode and the oxide semiconductor film;and forming a source electrode and a drain electrode over the oxidesemiconductor film, wherein the steps of forming the source electrodeand the drain electrode comprises the steps of: forming a first metalfilm; forming a second metal film over the first metal film; performinga first photolithography process on the second metal film and partlyremoving the second metal film by first etching; forming a third metalfilm over the first metal film and the second metal film to cover thesecond metal film; and performing a second photolithography process onthe third metal film and partly removing the first metal film and thethird metal film by second etching, and wherein the second etchingpartly remove the first metal film and the third metal film at an outerside of end portions of the second metal film which is removed by thefirst etching.
 2. The method for manufacturing a semiconductor device,according to claim 1, further comprising the steps of: forming a firstinsulating film over the source electrode and the drain electrode;introducing oxygen into the first insulating film; forming a secondinsulating film over the first insulating film; forming an aluminum filmover the second insulating film; introducing oxygen into the aluminumfilm to form an aluminum oxide film; and forming a planarizationinsulating film over the aluminum oxide film.
 3. The method formanufacturing a semiconductor device, according to claim 1, wherein eachof the first metal film and the third metal film is a metal film or ametal nitride film containing one or more elements selected fromtungsten, tantalum, titanium, and molybdenum.
 4. The method formanufacturing a semiconductor device, according to claim 1, wherein thesecond metal film contains a copper element.
 5. The method formanufacturing a semiconductor device, according to claim 1, wherein thefirst etching is a wet etching method.
 6. The method for manufacturing asemiconductor device, according to claim 1, wherein the second etchingis a dry etching method.